In certain tactical situations, military aircraft are required to operate at very low altitudes, in the so-called nap of the earth (NOE) mode in order to escape detection by taking advantage of the masking effect of nearby hills and trees. In this mode, helicopters often fly at altitudes of a few feet or even inches above ground level with their landing gear literally in the grass. Elongated objects such as wires and fences which can be difficult to see even in clear weather present a serious hazard to such flight modes. The present invention is a sensor or detection system especially designed to detect such elongated obstacles in the aircraft's path and to provide the pilot with an appropriate warning. The system is designed to be packaged in a pod for attachment to the aircraft's fuselage.
The detection system comprises an optical radar operating in the 10 micron wavelength region with a modulated transmitter laser and a continuous wave (cw) local oscillator laser arranged for heterodyne detection of target echoes. The apparatus includes a scanner for directing the radar beam in a desired pattern forward of the aircraft, a Brewster angle duplexer, beam combiner and distributor optics, beam expanding and contracting optics, cryogenically cooled detectors, a receiver including signal processing circuits and warning and alarm circuits.